A lot of Diamond and Pearl's problems were along the liens of running terribly, bad writing and a lackluster pokedex. All of these things are things that modern-day Gamefreak seem to be on the ball about, so there isn't too much I'm particularly worried about.But still...
>Many Mega Evolutions. I'm fine with a number in between XY and ORAS
>In terms of Megas, Fuck Ubers to hell and back with the Creation Trio and Arceus, and make a bunch of the Megas give the Ultra Beasts the heebee jeebees
>Use the Platinum pokedex. Considering that pokemon games today are using bigger and bigger pokedexes, this shouldn't be an issue
>Fuck the Battle Frontier for all I care, just give the Battle tree more characters from across the series
>Expanded story ala ORAS. Platinum is one of those games where they tried to write a story, and as a result it's one of the weaker ones.
>More Delta Episode like epilogues involving different legendaries. One with Arceus and Giritina, one with Darkrai and Creselia, etc.
>Walking with your pokemon